Continuous characters in phylogenetic analyses: Patterns of corolla tube length evolution in Lithospermum L. (Boraginaceae)

Abstract

The present study comprises an analysis of six different scoring schemes and eight different types of analytic methods aiming to investigate the evolution of a continuous character (i.e. corolla tube length) in Lithospermum L. (Boraginaceae). Corolla tube length in the genus is quite variable, ranging from 1mm to 75mm, and the length of the corolla tube has implications for pollination biology, such as longer corolla tubes (>25mm in length) being pollinated by hummingbirds or moths. In general, the various methods resolve similar ancestral character states; however, different states are reconstructed at nodes in which the descendants greatly differ in corolla tube length. Additionally, it is suggested that all of the variation of a continuous character should be included in analyses, and this may necessitate multiple analyses with different partitions of the data. The various analyses provide evidence that two maximum parsimony methods, linear parsimony and the TNT method, minimize the number of different rates of evolution. In Lithospermum, six origins of corolla tubes >20mm in length are resolved, and these origins occurred at two different times periods: (1) in the shadow of hummingbird diversification in North America (approximately 6-8Mya) and (2) more recently (approximately 1-1.5MyA). Four substantial decreases in corolla tube length also are reconstructed, and these may be associated with the origin of self-pollination. © 2012 The Linnean Society of London.